To combine good performance with low quiescent current, many amplifier circuits are realized with a class-AB output stage. Furthermore, it is desired to have a second amplifier stage with high gain, to obtain low distortion. High gain is typically realized by using a common source type of circuit, or common emitter in case of bipolar transistors. In a compact, two-stage amplifier, this means that class-AB behavior is wanted in a common source type of circuit. The combination of common source output transistors and class-AB behavior can be realized by using one of the feed-forward circuits as presented in references [1], [2], [3], [4], [5], [6], and [10] listed below.
Also feedback circuits can be used which typically control the quiescent current more accurately and can give more symmetric behavior, which is good for distortion performance. Examples of these are presented in references [1], [2], [7], [8], [9], [10], and [11]. These circuits make use of a minimum selector, which compares the minimum current in the output stage transistors with a reference current. A feedback loop then ensures that the minimum current will stay at the reference level, thus ensuring that none of the output transistors will switch off completely. The feedback loop allows for accurate control of the quiescent and minimum currents in the output stage.
The solution presented in references [1] and [2] uses the same type of minimum selector as some embodiments of the current invention, but it needs an extra stage for class-AB current control, in which the minimum current in the output stage transistors is compared to a reference current and the result is converted to a differential signal that is fed back to the output stage transistor.
The solutions presented in references [8], [9], and [10] use other types of minimum selectors, but they again need many extra transistors to compare the minimum current with the reference current and then converting the result into a differential signal that can be fed back to the output transistors.
In reference [11] not the minimum current is controlled but the product of the two currents in the output transistors is kept constant. This also ensures that none of the output transistors will be switched off completely.
The solution presented in reference [7] again uses an extra stage to convey the currents from the minimum selector to the output transistors.